The Germans had created one of the strongest ciphers of seen with the invention of the enigma. If they had kept standard practices and routine shifts in keys the cipher would have been impossible to decipher. The Germans originally planned on continued randomization of the day keys and plugboard settings. As the war went on, the randomness decreased and they began to use the same day keys and with more of a pattern. The Allied forces were able to build on the Polish achievements and spot these patterns, or cillies as they called them. Often times the same key would be used repeatedly just for the sake of easiness, disregarding the need for more security.
Another point of weakness in the German enigma was that methods in which they believed made their machine more secure actually made it easier to decrypt. Their idea that never making a swap between adjacent letters actually eliminated two possibilities on the plugboard. Also, never allowing a scrambler to be in the same spot two days in a row made sure that the same number was never repeated or that a scrambler was never in the same place twice. The codebook compilers reduced by half the number of possible scrambler arrangements as stated in The Code Book. This greatly reduced the combinations that codebreakers at Bletchley Park had to attempt.
The Germans had an impenetrable cipher that was flawed with human error. The Allieds Tipex and SIGABA ciphers both went unbroken throughout the entire war due to their increased complexity and diligence with sticking to proper protocol.